The present invention is directed to the aircraft flight control art and, in particular, to an active control system for suppressing aircraft body accelerations due to natural mode excitations.
Large aircraft, particularly those having a high length to diameter ratio, are subject to gust induced lateral accelerations of the tail section as a result of excitation of natural modes of the aircraft body. These lateral accelerations are particularly objectionable in commercial aircraft since they can result in the discomfort of aft seated passengers.
While modern commercial aircraft employ flight control yaw dampers to reduce yawing rates, such yaw dampers lack the capability to suppress flexible body mode accelerations. For aircraft in which the slenderness ratio (i.e., ratio of length to diameter) is not excessive, techniques employing a yaw damper operating at the Dutch Roll frequency and a notch filter at the location of the first body mode have proved successful in not exciting the structural osillation. Such systems are generally referred to as passive methods for controlling structural oscillation.
However, for aircraft having a higher slenderness ratio, and thus exhibiting a more severe oscillation in which the frequency of oscillation changes with varying flight conditions an active modal suppression system is required. One such prior system employs information from the onboard inertial reference system (IRS) to determine yawing of the aircraft and thereby develop a signal which, when passed through the yaw damper, is suitable for deploying the rudder to counteract the effect of the induced oscillation. This system is inadequate in high slenderness ratio aircraft as the signal sensed by the IRS, and the control law used therewith, do not fully define the accelerations experienced by the aft section of the aircraft structure.
There is a need in the aircraft flight control art, therefore, for an active modal suppression system which is capable of accurately monitoring aircraft structural accelerations due to excitation of natural aircraft modes and an associated control system to deploy aircraft control surfaces for suppressing aircraft body deflections due to modal oscillations.